As much as twenty percent of the population has some sort of hearing difficulty. It is typical for persons over 50 years of age to experience progressive loss in their aural perception in the high frequency part of the audio spectrum. A large percentage of those who have hearing impairment are aided in their understanding of speech in face-to-face communications by their familiarity with visual cues, and because the other persons speaking to them will adjust the loudness of their voices.
However, visual cues are not available to the hearing impaired listener in a telephone conversation, and non-verbal interaction between communicants on the telephone is not possible. Also, there is from time-to-time the added problem of telephone noise and speech signal distortion which will add to the problems of the hearing impaired.
Moreover, many of those with hearing impairments do not have hearing aids. Even those hearing impaired persons who have hearing aids may have problems when attempting to use the hearing aid with a telephone due to feedback occurring because of the close proximity of the telephone receiver and hearing aid microphone, and difficulty in maintaining the optimum position of the telephone receiver. It is not uncommon for someone to have a hearing aid fitted to their best ear, but because of the problem of hearing aid--receiver interaction, the person uses the other ear for telephone communications.
It is known that the speech spectrum exists mainly in the band below 8,000 Hz, and that the most important region lies below 5000 Hz. Most of the power of the signal is contained in the band 100 to 1000 Hz, while the middle to higher frequencies contribute significantly to the intelligibility of the signal. The speech signal has a great deal of redundancy, in fact the band below 1500 Hz has about the same amount of intelligibility as the band above 1500 Hz. The telephone signal capitalizes on this redundancy and uses a band of 300 to 3200 Hz for voice signals.
While for the average person the telephone signal typically gives an intelligibility of better than 90%, for a significant minority of the population who have hearing impairments the telephone signal can present varying degrees of intelligibility.
At each frequency level within the telephonic bandwidth, the hearing characteristics of a particular listener may be measured by two parameters. First, is the threshold value ("T") which indicates the power level that each frequency point must have for the listener to be able to hear that particular frequency. Second, is the limit ("S") on the listener's dynamic range at each frequency point, which indicates when the listener will experience pain or discomfort when the power level at the frequency point is increased.
The T and S values constitute a hearing profile which characterizes an individual listener. These profiles may commonly grouped or classified to match typical hearing impairment problems. Alternatively, the hearing profile of any particular listener may be unique to the aural impairment, disorder or disease suffered by that listener. Both the typical classifications of hearing impairment profiles and the unique hearing impairment profiles may be recorded and stored in a database for retrieval for adaptive processing of speech signals in the manner provided by the present invention.